Photochemie des (Iso) Alloxazins, III. Intramolekulare Photodealkylierung von 10‐Alkylisoalloxazinen, eine Modellreaktion für den Riboflavin‐Photoabbau

Gladys, Monika; Knappe, Wolfgang‐R.

doi:10.1002/cber.19741071118

Cited by 26 publications

(6 citation statements)

References 39 publications

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“…The very complex course of this reaction, yielding lumiflavin at alkaline pH and lumichrome at neutrality, is still not completely resolved [2] (and references therein). This is largely due to a common mistake of insisting that such reactions must involve radical Dedicated to Professor Hellmut Bredereck, Universitat Stuttgart, on his 75th birthday.…”

Section: (111)mentioning

confidence: 99%

Distinction of 2e- and le- Reduction Modes of the Flavin Chromophore as Studied by Flash Photolysis

Hemmerich

Knappe

Kramer³

et al. 1980

Eur J Biochem

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Evidence is given for the fact that the excited flavin triplet (3F10*x) exhibits competitive le-and 2e-transfer chemistry, depending on the nature of the photosubstrate. As an 'external' photoreductant, the 2e-donor borohydride has been investigated. Borohydride is found to compete effectively with the 'internal' 1 e-donors, namely excess starting flavin in the ground state (Flax) and, as primary product, (alky1)dihydroflavin (RFl,,dH). It will be shown that some of the flavin radicals, observed by earlier authors in the reaction of 3Fl& with CH or C -COO-substrates or in the autophotolytic side-chain cleavage of riboflavin, are due to the dye-dye reaction (I) :(1) H i In contrast flavin photoreduction by borohydride or hydrocarbon substrates need not involve radicals, but may in fact be a hydride or 'carbanion-plus-proton' addition towards the highly polar and considerably basic (pK = 4.4) acceptor triplet (cf. reaction 11) RCOO-The products are much more photoreactive than the starting substrates, which leads to the secondary photocomproportionation (111) : 3Fl,*, + R-Fl,,dH + HFl + RFl, (R = alkyl or H). (111) This latter reaction is the second source of radicals in the system. This (cf. 11) 'photohydrogenation' of flavin is mechanistically related to the biological reduction of flavin by CH substrates.The photochemistry of flavins is in fact older than the corresponding ground-state chemistry and even biochemistry, since the side-chain photolysis of vitamin Bz was the first flavin reaction to be discovered [l]. The very complex course of this reaction, yielding lumiflavin at alkaline pH and lumichrome at neutrality, is still not completely resolved [2] (and references therein). This is largely due to a common mistake of insisting that such reactions must involve radical Dedicated to Professor Hellmut Bredereck, Universitat Stuttgart, on his 75th birthday.Abbreviations. Fl,,, flavoquinone, flavin; 3Fl,*,, excited flavin triplet; HFI Fl-, acidic and basic flavosemiquinone; Fl', oxidized flavin radical; HZFlred, flavohydroquinone, 1,5-dihydroflavin.

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Section: (111)mentioning

confidence: 99%

Distinction of 2e- and le- Reduction Modes of the Flavin Chromophore as Studied by Flash Photolysis

Hemmerich

Knappe

Kramer³

et al. 1980

Eur J Biochem

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“…(4) Photo-dealkylation at N-10. The long-known photo-dealkylation at N-10 has been shown to be a fragmentative process encountered only with N'0-alkyl residues bigger than ethyl (Gladys & Knappe, 1974). This is the only photolytic reaction that would not yield bleaching, i.e.…”

Section: C6h5ch2oh and C6h5c2h20hmentioning

confidence: 99%

Flavin-dependent substrate photo-oxidation as a chemical model of dehydrogenase action

Haas¹,

Hemmerich²

1979

Biochemical Journal

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As a model of flavin-dependent biological dehydrogenation, flavin-sensitized photodehydrogenation and photodecarboxylation were studied by variation of substrate, flavin, pH and solvent. Evidence for the following rules is given. (1) When the reactive site of a photosubstrate is an alpha-carbon atom of the type CH-CO2-, decarboxylation is preferred over dehydrogenation, whereas the reverse is true for the neutral CH-CO2H. (2) Consequently these reactions do not exhibit a measurable isotope effect with C2H-CO2-, in contrast with the findings by Penzer, Radda, Taylor & Taylor [(1970) Vitam. Horm. (N.Y.) 28, 441--466], which could not be reproduced. When the substate does not contain a carboxylate group, isotope effects occur, in verification of previous reports, e.g. for benzyl alcohol C6H5-C2H20H. (3) The mechanism of flavin-sensitized substrate photodecarboxylation is assumed to consist in a primary carbanion fixation at the flavin nucleus (position 4a, 5 or 8) with concomitant liberation of CO2. This step is followed by rapid fragmentation of the adduct CH-Fl-red., provided that the substrate contains a functional and electron-donating group X, e.g. X = OH, OCH3 or NH2 (but not NH3+ !) in X CH-CO2-. (4) The minimal requirement for flavin-sensitized C-H dehydrogenation is the presence of a hydroxyl group. For example, methanol as substrate and solvent is dehydrogenated at pH sufficiently alkaline for detection of the presence of the active species CH3O-, whereas at more acidic pH substrate dehydrogenation is competing with flavin autophotolysis, which depends on the substituents in the flavin nucleus.

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“…This is substantiated by a decrease in fluorescence emission of RF solutions in the presence of these species. The participation of the 1 RF state in the intramolecular photodegradation of RF leading to the direct formation of LC has been reported (44,(57)(58)(59)(60) (62,63). There is no significant decrease in the fluorescence of RF solutions in the presence of carbonate ions (Table II).…”

Section: Fluorescence Studiesmentioning

confidence: 83%

Effect of Acetate and Carbonate Buffers on the Photolysis of Riboflavin in Aqueous Solution: A Kinetic Study

et al. 2014

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Abstract. The photolysis of riboflavin (RF) in the presence of acetate buffer (pH 3.8-5.6) and carbonate buffer (pH 9.2-10.8) has been studied using a multicomponent spectrophotometric method for the simultaneous assay of RF and its photoproducts. Acetate and carbonate buffers have been found to catalyze the photolysis reaction of RF. The apparent first-order rate constants for the acetate-catalyzed reaction range from 0.20 to 2.86 ×10 −4 s −1 and for the carbonate-catalyzed reaction from 3.33 to. The second-order rate constants for the interaction of RF with the acetate and the carbonate ions range from 2.04 to 4.33×10 −4 M −1 s −1 and from 3.71 to 11.80×10, respectively. The k-pH profile for the acetate-catalyzed reaction is bell shaped and for the carbonate-catalyzed reaction a steep curve. Both HCO 3 − and CO 3 2− ions are involved in the catalysis of the photolysis reaction in alkaline solution. The rate constants for the HCO 3 − and CO 3 2− ions catalyzed reactions are 0.72 and 1.38×10, respectively, indicating a major role of CO 3 2− ions in the catalysis reaction. The loss of RF fluorescence in acetate buffer suggests an interaction between RF and acetate ions to promote the photolysis reaction. The optimum stability of RF solutions is observed in the pH range 5-6, which is suitable for pharmaceutical preparations.

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Photochemie des (Iso) Alloxazins, III. Intramolekulare Photodealkylierung von 10‐Alkylisoalloxazinen, eine Modellreaktion für den Riboflavin‐Photoabbau

Cited by 26 publications

References 39 publications

Distinction of 2e- and le- Reduction Modes of the Flavin Chromophore as Studied by Flash Photolysis

Distinction of 2e- and le- Reduction Modes of the Flavin Chromophore as Studied by Flash Photolysis

Flavin-dependent substrate photo-oxidation as a chemical model of dehydrogenase action

Effect of Acetate and Carbonate Buffers on the Photolysis of Riboflavin in Aqueous Solution: A Kinetic Study

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